1. Field of the Invention
The present invention relates to feeder chucks used in machine tools to advance a bar to be machined when it is introduced into a gripper chuck.
In a machine tool, the portion of bar to be machined is held by a gripper chuck which holds it firmly during machining by the tools of the machine tool. Machining consists of removing material from a section of bar whose length is equal to that of the part to be made.
Between machining two successive parts, the part that has just been made is ejected, and the bar must be advanced to introduce its end into the gripper chuck. The bar is advanced by a feeder chuck which grasps the bar sufficiently by friction to overcome all forces resisting the advance of the bar and inserts it into the gripper chuck. The feeder chuck is mounted on a carrier which slides longitudinally in the machine tool relative to the gripper chuck.
2. Description of the Prior Art
Machine tool feeder chucks generally include a tubular chuck body having an axial bore through which a bar to be introduced into the machine tool is passed, and which has a downstream chuck body section which can deform elastically in the radial direction to bear elastically on the outside surface of the bar. The bar is forced into the feeder chuck, for example by a bar feeder that pushes it in the longitudinal direction, and the bar spreads the branches of the feeder chuck apart elastically. The stiffness of the branches of the feeder chuck must be chosen to grip the bar appropriately in the radial direction to provide a particular axial holding force between satisfactory minimum and maximum values. For the most usual bar diameters an axial bar holding force from 400 to 600 Newtons is required, for example.
A feeder chuck of the above kind is not adjustable. The problem is that the axial retaining force that it applies to the bar essentially depends on the diameter of the bar. In mass production, the bar diameters can differ slightly from one batch to another, which induces a variation in the axial holding force applied by the feeder chuck which can be outside the tolerance limits for the axial holding force.
To avoid this problem, adjustable feeder chucks were proposed many years ago, like those described in the document DE 2 405 527 A, for example. The tubular chuck body described in that document further includes an exterior envelope having an internal clamping conical bearing surface that is adapted to come into contact with via corresponding external conical clamping bearing surface of the chuck body, and has an interior screwthread section co-operating by means of a screwing action with an exterior screwthread of the chuck body to clamp the chuck body downstream section radially against the exterior surface of the bar. The envelope includes longitudinal slots into which lugs projecting from the exterior face of the chuck body can be inserted to prevent the exterior envelope rotating relative to the chuck body. A special tool is used to clamp the chuck body radially and to drive rotation of the chuck body in the exterior envelope, to modify the longitudinal position of the chuck body relative to the exterior envelope by means of a screwing action and thereby modify the clamping of the bar.
This kind of device has the disadvantage that it necessitates a special tool for clamping the chuck body radially and allowing it to rotate for the adjustment by screwing so that the immobilising lugs are withdrawn from the immobilising slots in the exterior envelope. Also, the adjustment is discontinuous because the lugs necessarily have to engage in one of the longitudinal slots of the exterior envelope. Further, this structure cannot be applied to parts to be machined which have a small diameter, because the proportion of the diameter occupied by the chuck to withstand mechanical forces is then too large to enter the corresponding housings of the machine tool. This kind of arrangement is even more inapplicable to chucks whose bearing surface is made of carbide, given the necessary overall size of the carbide part.
It has also been proposed to make the clamping surface of feeder chucks from carbide, i.e. from a sintered mixture of tungsten carbide, cobalt and nickel, because this kind of structure slides freely on steel and prevents the binding that is sometimes observed if a steel chuck is used. However, prior art chucks with a carbide clamping surface, as described in the document EP 0 813 923 A, for example, are not adjustable in diameter, and scratch some bars because the elastic travel of the chuck is too great if the bar has a large diameter.
The problem addressed by the present invention is that of proposing a new adjustable feeder chuck structure, designed to reduce the production of scratches on the bars and to increase its service life by limiting rubbing on the bar.
Another object of the invention is to design a feeder chuck structure of this kind that can be used even for small bar diameters, offers continuous diameter adjustment, requires no special adjuster tool, and is very convenient to adjust on the machine tool.
Another object of the invention is to improve the holding of the bar, by providing rigid and symmetrical holding of the bar centred facing the gripper chuck, with no possibility of splaying or off-centring.
To achieve the above and other objects, a machine tool feeder chuck in accordance with the invention includes:
a tubular chuck body having an axial bore through which a bar to be inserted into the machine tool is passed,
a screwthreaded upstream section for fixing the chuck to a feeding carriage of the machine tool,
a chuck body downstream section, including longitudinal slots so that it has a capacity for elastic deformation in the radial direction enabling it to bear on the exterior surface of the bar, and including an external conical clamping bearing surface,
an exterior envelope, having a downstream section with an internal conical clamping bearing surface in contact with the external conical clamping bearing surface of the chuck body, and having an upstream section with an interior screwthread meshing with an exterior screwthread of the chuck body to clamp the downstream section of the chuck body in the radial direction;
in accordance with the invention:
the screwthreaded fixing upstream section of the chuck is fastened to the chuck body,
the exterior envelope has a structure having a high resistance to radial expansion in its downstream section with the internal conical clamping bearing surface,
the exterior envelope includes means conferring on it a capacity for elastic deformation in the longitudinal direction in the part of its length between the internal conical clamping bearing surface and the interior screwthread.
The operation of a feeder chuck of the above kind in accordance with the invention differs from the operation of a prior art adjustable chuck.
In prior art adjustable chucks, to preserve the capacity for radial elastic deformation of the chuck on inserting the bar or when it slides in the feeder chuck, the area clamped by the conical clamping bearing surfaces is offset axially relative to the area of the bar on which the chuck body bears. Thus, the elasticity of the feeder chuck is provided by members in the form of longitudinal tongues that flex in the radial direction independently of each other. If the bar applies a radial force to the feeder chuck, only a portion of the periphery of the feeder chuck holds the bar centred. This results in elastic holding which lacks stiffness.
In contrast, with the structure according to the invention, the capacity for elastic deformation in the radial direction, that is necessary for inserting the bar or for it to slide relative to the feeder chuck, is provided not by the downstream section of the chuck body with longitudinal slots, but by the capacity for elastic deformation in the longitudinal direction of the exterior envelope, whose structure also has a high resistance to radial expansion. Consequently, when the bar applies a radial force to the feeder chuck, the whole of the stiffness of the exterior envelope, rather than only a portion of its periphery, contributes to holding the bar centred. The feeder chuck structure in accordance with the invention therefore achieves much more effective centred holding of the bar against lateral oscillations.
At the same time, the adjustment of this kind of structure is continuous, and requires no special tool such as a clamping tool to allow relative rotation of the chuck body in the exterior envelope.
The longitudinal portion of the chuck body directly facing the internal clamping conical bearing surface of the exterior envelope preferably bears radially on the bar.
In a first embodiment, the clamping conical bearing surfaces have their apex oriented in the upstream direction.
In a preferred embodiment, the clamping conical bearing surfaces have their apex oriented in the downstream direction.
In one advantageous embodiment that is efficient, economical and easy to manufacture, the means conferring a capacity for longitudinal elastic deformation include elongate openings oriented obliquely to the longitudinal axis of the exterior envelope and distributed around the longitudinal axis in the exterior envelope portion between the internal conical clamping bearing surface and the interior screwthread.
Good results have been obtained by providing elongate openings inclined at an angle from approximately 5xc2x0 to approximately 12xc2x0 to the longitudinal axis. An inclination of about 8xc2x0 would appear advantageous.
The elongate openings can have a width similar to that of the oblique longitudinal members that flank them. Their number is chosen according to the required stiffness characteristics. Four to six elongate openings may be suitable for the most usual chuck diameters.
In one convenient embodiment, the means for preventing the exterior envelope from rotating relative to the chuck body can include a lock-nut screwed onto the exterior screwthread of the chuck body.
In one advantageous embodiment, the external conical clamping bearing surface of the chuck body is formed on the downstream end face of the chuck body.
In this case, the external conical clamping bearing surface of the chuck body can advantageously be relieved by reducing the diameter in the vicinity of the downstream ends of the longitudinal slots.
Other objects, features and advantages of the present invention will emerge from the following description of particular embodiments, which description is given with reference to the accompanying drawings.